1. Field of the Invention
The present invention relates to an apparatus for detecting a toner concentration (composition ratio) in developing powder in an electrostatic printing machine which uses the developing powder comprising a mixture of magnetic carrier and toner.
2. Description of the Prior Art
In an apparatus in which an electrostatic latent image is developed by developing powder comprising the mixture of the magnetic carrier and the toner, only the toner is consumed in the developing process but the magnetic carrier is not substantially consumed but it is maintained for reuse. As a result, as the developing powder is used repeatedly, a toner concentration of the developing powder decreases gradually. In order to maintain a high quality of the developed image, however, the toner concentration should be maintained at a fixed value or in a given range. Accordingly, it is necessary to detect the toner concentration. It has been proposed to utilize a change in permeability of the developing powder in order to detect the toner density, an example of which is U.S. Pat. No. 3,572,551 "Apparatus for Monitoring and Controlling the Concentration of Toner in a Developer Mix" to Henderson et al, application Ser. No. 811,132 filed Mar. 27, 1969, issued Mar. 30, 1971 and assigned to RCA.
In the conventional apparatus, however, a separate drive means for forcibly recirculating the developing powder has been required in addition to the developing apparatus, and the accuracy of detection was low when the toner concentration was measured in such a forcibly recirculated system.